Waveguide gratings are conventionally fabricated by doping a waveguide core with one or more photosensitive dopants. Upon illumination with the appropriate wavelength light, typically ultraviolet, a permanent refractive index increase is produced in the core. The appropriate periodic spacing of perturbation to achieve a conventional grating can be obtained by the use of a phase mask or an amplitude mask.
Long period fiber gratings devices provide wavelength dependent loss and can be used for spectral shaping. The grating provides coupling between two copropagating modes of the waveguide with very low back reflection. Long period fiber gratings typically have periods which are at least 10 times larger than the transmitted wavelength, i.e.: .LAMBDA.&gt;10.lambda. and are usually in the range of 15-1500 filters light in a narrow bandwidth centered around the peak wavelength of coupling, .lambda..sub.p, determined by: EQU .lambda..sub.p =(n.sub.01 -n.sub.lm).LAMBDA.,
where n.sub.01 and n.sub.lm are the effective indices of the fundamental and cladding modes, respectively. Typical bandwidths are in the range of 2-50 nm. The value of n.sub.01 is dependent on the core and cladding refractive indices, while n.sub.lm is dependent on core, cladding and ambient indices.
An important application of long period gratings is to flatten the gain spectrum of an erbium-doped fiber amplifier. Flattening is achieved by cascading one or more gratings to give the desired spectral response. Alternatively flattening can be achieved by chirping the grating period .LAMBDA..
Under certain circumstances the effective gain spectrum of an amplifier can vary in time due to a combination of effects. In particular, as the number of channels passing through the amplifier changes, the amplifier exhibits deleterious peaks in its gain spectrum requiring modification of the long period grating used to flatten the amplifier. In this case a reconfigurable grating would be desirable. Unfortunately conventional gratings are essentially permanent and generally not reconfigurable. Thus there is a need for gratings which can be controllably reconfigured in bandwidth.
It is known that heating a grating causes a shift in the resonant wavelength. For a typical long period fiber grating, the shift is of order 5 nm for a 100.degree. C. temperature change. A similar effect can be achieved in a Bragg grating whereby a 100.degree. C. temperature gives rise to a 1 nm shift. It has also been recognized that a temperature gradient along the length of a Bragg grating will broaden the bandwidth of the spectrum. Such a "chirped" Bragg grating can serve as a dispersion compensator. However, because temperatures typically must be kept well below 500.degree. C. to avoid detrimental effects on the glass, the largest shift obtained is about 5 nm.
For many applications, such as gain flattening, the amount of broadening obtainable by heating is insufficient as it would not cover the erbium gain bandwidth of approximately 40 nm. And as larger bandwidths are employed, additional broadening will be required. Thus there remains a need for optical grating devices with reconfigurable bandwidths.